The invention relates to a heating device for heating a yarn, in particular in a yarn texturing machine.
For texturing a synthetic filament yarn, it is known to heat the yarn in a texturing machine by means of a heating device for texturing and drawing and for subjecting the yarn to a shrinkage treatment. As a function of the process step, different heating devices have been developed, which produce an optimal heating of the yarn in the particular process. Thus, for obtaining a high relaxation effect in the yarn during the shrinkage treatment, it is necessary that the yarn advance through the heating device as much as possible under no tension. For this process step, especially such heating devices have proved successful, in which the yarn advances through a yarn channel that is provided within a heating tube. Heating devices of this type are known, for example, from DE 23 48 371, or DE 31 01 925 A1. In these heating devices, the heating tube is formed by a thin-walled tube, which is surrounded by a heating jacket that is filled with a heat transfer medium. For guiding the yarn, the heating tube accommodates a tubular insert, which includes a yarn channel. In this process, the yarn is heated by convection, preferably in a noncontacting relationship. However, devices of this type are also known for heating the yarn in contacting relationship.
Basically, with such heating devices there arises the problem that the heat generated on the circumference of the wall of the heating tube by the condensation of the heat transfer medium, must be guided through the wall of the heating tube and through the wall of the insert for entering the yarn channel. To this end, it is necessary to fit the tubular insert into the heating tube as tightly as possible without leaving any gaps, so as to avoid heat transfer losses. However, since it is preferred to mount the inserts in the heating tube in an exchangeable manner, it is, for example, not possible to press them in for obtaining narrow gaps and satisfactory heat transfers.
When texturing and drawing yarns, it is preferred to use heating devices, as are disclosed, for example, in EP 0 412 429 A1 and corresponding U.S. Pat. No. 5,148,666. During the texturing and drawing processes, the yarns advance under a high tension while being heated. In this connection, the yarns exhibit natural dynamics, so that the yarn guidance becomes an exacting task. Heating devices of this type are preferably constructed as high-temperature heaters, in which the yarns advance in a heating groove with yarn guide elements. The sides and the bottom of the heating groove are heated by resistance elements, with the heating surface having a temperature that is above the melt point of the yarn material. For this reason, the yarns are kept at a distance from the heating surface. However, to heat a yarn in a tension free state, such heating devices are barely suitable, since each unintended contact with one of the heating surfaces directly results in damage to filaments of the yarn.
It is therefore an object of the invention to further develop a heating device of the foregoing type for heating a yarn, in particular during a shrinkage treatment in a texturing machine, such that it permits transferring heat into the yarn channel with the least possible losses.
A further object of the invention is to achieve in the yarn channel the highest possible yarn temperatures while using the energy generated by a heating jacket.